Optical resin materials which are characterized by a distributed refractive index have demonstrated usefulness in the construction of optical conductors such as, optical fibers, optical waveguides, and optical integrated circuits as well as the corresponding preforms of these conductors. In general, plastic optical fibers (POF) are considered an attractive alternative to copper cable and glass optical fibers. Typically, the plastic optical fiber (or thin, flexible rod) has a core within which light travels and a sheathing layer which surrounds the core, confines the light to the core and possesses an index of refraction less than that of the core.
The refractive index distribution of plastic optical fibers can be classified as either gradient index or step index. However, graded index plastic optical fibers (GI POF) are preferred over step index fibers for data communication applications. That is, the index of refraction, in a graded index plastic optical fiber, generally decreases radially from the core center outward until it matches the sheathing index at the core-sheathing interface. Therefore, light rays entering the core at a small angle, with respect to the axis, follow undulating paths, which is not the case for a step index type fiber. The speed of the light rays along the undulating paths increases in the regions of lower refractive index so that the travel time along these paths is nearly equal to that along the straight axial path. This results in, for example, a wider bandwidth of transmission with minimal modal dispersion and a more rapid information flow than that obtained with step index plastic optical fibers.
In general, methods of fabricating graded index plastic optical materials comprise preparation of a polymeric sheathing and a polymeric core disposed within the sheathing. The refractive index of the core and sheathing are different in that the refractive index of the core is greater than that of the sheathing. Frequently, the core is the same polymer as that which comprises the sheathing but, in addition includes a non-polymeric substance (commonly referred to as a dopant) which causes the refractive index of the core to be greater than that of the sheathing. See for example, U.S. Pat. No. 5,541,247 to Koike.
However, currently available methods of fabrication have significant shortcomings. For example, the type and amount of substances which can be incorporated into the core and still provide a graded index plastic optical material which maintains both transparency and an acceptable difference in the refractive index between the sheathing and the core, are limited. Therefore, a need exists for methods and materials useful for fabricating graded index plastic optical materials.